The present invention relates to polyacetal compositions containing certain compounds selected from epoxy-containing polymer compounds, oxazolyl-containing monomer and polymer compounds, styrene acrylonitrile graft polymer compounds, and mixtures thereof. The compositions exhibit better long-term heat aging properties and better resistance to grease than polyacetal alone.
Polyacetal resin is manufactured by polymerizing a mostly formaldehyde monomer or a formaldehyde trimer (i.e., trioxane). Examples of polyacetal resin include acetal homopolymer (for example, DuPont Company's Delrin.RTM.) and acetal copolymers, which are obtained by copolymerizing formaldehyde and ethylene oxide.
Polyacetal resin, because of its excellent mechanical properties (such as outstanding repeated fatigue characteristics, abrasion resistance, and wear resistance) and electrical properties is extensively used in automotive and electrical/electronic parts. However, it has been found that, in some cases, the conventional polyacetal resin may not have satisfactory long-term resistance to heat aging (i.e., long-term heat stability) and grease resistance (i.e., long-term oil resistance). With respect to heat aging, it has been found, in some cases, that the conventional polyacetal resin, when exposed to high temperatures of about 120.degree. C. or above over an extended period of time, may fail to maintain certain mechanical properties, such as tensile strength. With respect to oil resistance, it has been found, in some cases, that the conventional polyacetal resin, while resistant to gasoline and lubricants at moderate temperatures (about 80.degree. C.), may behave as if corroded by gasoline and lubricants at higher temperatures (about 100.degree. C. or above).
U.S. Pat. Nos. 4,098,843 and 4,845,161 teach ways to improve on the long-term aging characteristics of polyacetal by incorporating therein antioxidants or heat stabilizers (for example, polyamides or polycarbodiimides), but sufficient improvement in properties has not been attained.
Thus, the present invention is aimed at providing a polyacetal resin composition with improved resistance to long-term heat aging and to long-term grease exposure. Extensive studies by the present inventors have led to the discovery that incorporation into a polyacetal resin of certain compounds selected from epoxy-containing polymer compounds, oxazolyl-containing monomer and polymer compounds, styrene acrylonitrile graft polymer compounds, and mixtures thereof, provides a polyacetal resin with improved resistance to long-term heat aging and to long-term grease exposure, as compared to polyacetal alone.